1. Field of the Invention
The present invention relates to an improvement of an electronically controlled transmission of a type having a shift rod operated through a shift drum.
2. Description of Related Art
Transmissions are typically provided with shift rods which are selectively shifted by means of a shift lever to place the transmission in desired gears. Especially, electronically controlled transmissions are equipped with shift drums which are driven by an electric actuator. The shift drum is formed with cylindrical cam grooves with which shift rods are engaged. Gear shift operation of the shift lever causes the actuator to rotate the shift drum, causing a selective shift of the shift rods so as to place the transmission into a gear selected by the shift lever. Such an electronically controlled transmission is known from, for instance, Japanese Unexamined Patent Publication 64-46049.
One of requirements for rear-drive vehicles is to miniaturize the transmission so that it is as small as possible because the transmission is located in a small space under a floor tunnel. Though it is relatively easy to arrange the shift drum above the transmission mechanism because the shift lever is typically located in a passenger compartment, nevertheless, arranging the shift drum with its axis of rotation in parallel with and right above the transmission main shaft increases the overall height of the transmission. Such an arrangement of the shift drum leads to a bulky structure of the floor tunnel. If the transmission is brought down according to the overall height, it becomes difficult to provide a necessary ground clearance.
In order to reduce the size of a shift drum actuator, a rack and pinion mechanism is available in company with a hydraulic cylinder. In such a structure, an elongated rack must be contrived to be snugly arranged because of its long length.